OAHN Small Ruminant Network Project: Investigation of Pooling Serum Samples for Maedi Visna Testing (in progress)

Project Lead: Dr. Rex Crawford

Collaborators:  Dr. Jocelyn Jansen

The Ontario Maedi Visna Flock Status Program (OMVFSP) administered by Ontario Sheep Farmers recently completed an update to their protocols.  As part of that review, feedback was solicited from producers enrolled in the program.  One of the primary concerns of producers was the cost to complete whole flock individual animal testing – both in small and large flocks.  The participating producers encouraged the Technical Committee to allow individual serum samples to be pooled prior to testing with the Hyphen Elitest ELISA.  There was also a resolution passed at the Ontario Sheep Farmers annual meeting encouraging pooling as a way to get more producers to test for maedi visna.

Quebec has a maedi visna flock status program similar to the OMVFSP, where pooling has been allowed for a number of years.  The Quebec program allows 5 samples to be pooled and uses a cut-point Optical Density of 0.100.  Their validation study with 10 “high positive” and 25 “low positive” samples diluted with 4 previously negative samples indicated a loss of approximately 20% sensitivity.  Through the program, they find approximately 17% of pools are positive – with approximately 70% of those pools having at least 1 individual positive sample when retested individually.

The project plans to use samples submitted to the AHL, and tested previously, as the known individual samples.  Negative samples will be selected from submissions that have less than 10% positive samples to simulate low prevalence flocks.  Positive samples will be selected from multiple flocks.  Sample size calculations have been performed using assumptions from the Quebec dataset.  Sixty-two individual positive samples will be comingled with 2 negative samples and run on the Hyphen Elitest ELISA (Pool3+), and also comingled with 4 negative samples (Pool5+).  Eighty-one pools of 3 (Pool3-) and 5 (Pool5-) negative samples will also be run on the Hyphen Elitest ELISA.  In total, this will require 286 pooled samples to be tested and 62 known positive and 405 known negative samples.

Statistical analysis will be performed to select a cut-point (reported as Optical Density) for both the pools of 3 and pools of 5 that maximizes sensitivity without decreasing specificity greatly.  The closest corner method will be used to determine the optimal cutpoint, with further analysis performed if either sensitivity or specificity is deemed unacceptable at the optimal cutpoint.  Sensitivity and specificity for the selected cut-point will be communicated to the AHL and the technical committee of the OMVFSP for program consideration. Results of the project will be shared with veterinary practitioners and producers.